Difficulties are encountered in practice in the treatment of contaminated water flows such as sewage prior to fine filtration, due to the presence of heavy and generally untreatable objects such as rags, string, plastic bags, and the like. Wastewater treatment plants typically utilize some type of screening equipment to remove harmful debris contained in the waste stream flow. Screening equipment is often utilized in the headworks section of the plant, and is the first area to come in contact with the waste stream. The screens are typically made from corrosion resistant materials such as 304 or 316 stainless steel. In order to protect the downstream equipment and processes, screening equipment is designed and incorporated in a plant to remove a large majority of debris before it comes into contact with any downstream equipment. If such items are not removed, proper and adequate treatment of the liquid does not result, and blockage of ducts and channels may occur.
A screening device is known comprising a continuously movable endless conveyor loop formed of a series of interconnected link pieces each having a lifting hook on which material to be screened is collected during movement through the contaminated water flow. There are many different screening equipment designs. Fine screens are typically defined by the size of the screen openings, which can be from as small as a quarter inch to one-half inch. These units can also be sized to have clear openings as large as 2 inches or more, but typically are not. The screen openings are designed to address both the horizontal and vertical limiting dimensions. The horizontal dimension is the small dimension and definition of the continuous belt, for example, one-quarter inch. The vertical dimension is typically significantly larger (approx. 4 in. or 6 in.) and is tied to the length of individual elements and the interconnected driving links and support shafts or pivot rods.
An illustrative screen filter apparatus 10 is shown in FIGS. 1–3. Referring first to FIG. 1, it will there be seen that a waste material filtering apparatus 10 of the prior art generally includes a frame 11, a plurality of pipe spreaders 13, a drive motor 15 connected to a drive sprocket assembly 22 by a drive belt or chain 23, shown in phantom lines, and a rotating screen assembly 17 driven by the sprocket assembly. Referring to FIG. 2, the apparatus 10 sits in a channel 21 within which flows a stream of water containing solid waste.
The rotating grid assembly 17 includes a plurality of vertically disposed, laterally spaced apart rotating screen segments that collectively form the screen cleaning grid, with the lateral spacing between contiguous rotating screen segments limiting the size of the waste materials that can flow past the machine and into downstream treatment stations where smaller particles are removed from the water. Each rotating screen segment is formed by a plurality of link members 12 that are disposed in articulated relation to one another. The trailing end of each link 12 has an integral horizontally-extending part 16 that helps hold and lift solid matter from the stream as the screen segments travel upwardly on the upstream side of the machine. Means are provided at the discharge end of the apparatus for dumping the matter so lifted into a solid waste collection container.
The opposite end of each link 12 is mounted on a shaft 31, 33. The trailing end of each link 12 is the aforementioned horizontally-extending member 16 that helps hold and lift solid matter from the water stream as the links rotate, as is perhaps best understood by observing the links at the lower left corner of FIG. 1. The uppermost or leading end of each link is denoted 18. Plural directional arrows, collectively denoted 19, show the path of travel followed by the links as the machine operates. The orientation of machine 10 in a channel of water is shown in FIG. 2. The concrete channel is denoted 21. In this particular example, there are about twenty five upstanding screen segments disposed in equidistantly spaced lateral relation to one another, each screen segment being formed by a group of articulated link members 12.
The conventional assembly pattern of links is best understood in connection with FIG. 3. The links 12 of the prior art rotating screen are typically assembled in the following pattern. The trailing and leading ends 16, 18, respectively, of a link 12a are slipped onto a pair of contiguous shafts 31, 33, with the same procedure repeated about the remaining alternating shafts 31, 33 of the screen assembly 17. The trailing end of the next link 12b is then slid onto shaft 31 and the leading end of that link is slid onto shaft 33. The alternating pattern is then followed as links are placed on all of the shafts 31, 33. Spacers 20 are then added to each shaft, and the same pattern of assembly is repeated to construct a screen assembly 17 of a desired width. The alternating nature of the links 12a, 12b connects the links into a continuous loop.
The screen assembly 17 is generally a large structure that is directly driven by the sprocket drive assembly 22 and is under significant tension. Therefore, removal of any of the shafts 31, 33 after installation typically causes complete loop failure. Also, the tensile load also makes field reinstallation of the shaft difficult, if not impossible. Due to the continuous grid or belt design employed in these machines, the head tracking support structure of the grid, the driving sprockets and the wear tracking components in the submerged channel area are all located within the “continuous loop” of the grid and provide no operator access for either inspection or replacement of the wearing components. In order to perform an inspection of wearing components, the screen headsection must have the covers removed and the very heavy grids must be broken apart at various areas, generally one at a time, for inspections at key points, which is a complicated, tedious major maintenance activity. Also, this model of screen is prone to having wastewater screenings forced through the front grid and trapped by the rear return portion of the grid. Without flushing the trapped material out of the screen, the headlosses through the screen becomes excessive. In order to clean the grids out, high pressure hoses are trained on the mass of debris until eventually most is broken up and flushed out. However, with no simple means to flush the units this process becomes prohibitively time consuming to plant maintenance personnel. Accordingly, easy internal access for inspection and maintenance is desired.